Wireless voice and/or datacommunication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs). The term mobile station generally includes both hand-portable and vehicular mounted radio units. Furthermore, the communications link from a BTS to a MS is referred as to the down-link (or outbound) path. Conversely, the communications link from a MS to a BTS is referred to as the up-link (or inbound) path.
In a wireless communication system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area defines a particular range that the BTS can maintain acceptable communications with MSs operating within its serving cell. Multiple access techniques permit simultaneous transmissions from several MS to a single BTS over a plurality of communications channels.
Some channels are used for carrying traffic communications, whilst other channels (which may be logical or dedicated channels) are used for transferring control information, such as call paging, between the base transceiver stations and subscriber units. Examples of multiple access techniques include: frequency division multiple access (FDMA), time division multiplexing/multiple access (TDM, TDMA) and code division multiple access (CDMA).
Many data communication systems use contention mode as a means of allowing access to a shared communication channel/resource. In data communication systems operating contention mode access schemes, much effort has been directed at improving channel access by fast, fair and efficient means. Typical channel accessing techniques that exist are based on monitoring of the communication channel before contention. Some techniques are based on a use of control channels to allocate a particular time slot or frequency for a user wishing to gain access to a communication resource.
Referring now to FIG. 1, a Radio Data—Link Access Protocol (RD-LAP) channel contention mechanism 100 for an outbound channel in a data communication system is shown. The data communication system employs, for example, a Slotted Digital Sense Multiple Access (Slotted-DSMA) technique, to provide wireless data communication units (data modems) access to a communication resource, i.e. an inbound channel. The communication resource is shared amongst a number of data modems. A multiple access protocol is required to control and limit the amount of interference/collisions between the data modems, when requesting access to, or transmitting on, the communication resource.
An active data base (transceiver) station (DBS) continuously inserts Channel Status symbols 130, 132, 134, 136, 138 between outbound data transmissions 120 on the outbound channel 110. The DBS inserts the symbols in response to a determination of whether the corresponding inbound channel is ‘BUSY’ or ‘IDLE’.
One or more of the data modems is configured to observe these periodic channel status symbols 130, 132, 134, 136, 138 and make a decision on whether to contend or not for an access to the inbound channel.
The procedure for transmission of data packets 200, in the RD-LAP channel contention mechanism 100 of FIG. 1, is shown in FIG. 2. A data modem remains idle, in step 210, until a new protocol data unit (PDU) is received for transmission. A PDU relates to an information portion of a frame which includes address and control information, and optionally data. It is assumed that after an idle period, the data modem is not synchronised to the frame structure of the RD-LAP channel, and therefore enters a frame synchronisation mode, as shown in step 220.
Once frame sycnhronisation has been detected, the data modem waits a random time in order to decrease the risk of possible collisions with other units, in step 230. If the channel state is unknown, the data modem waits until it receives a channel state symbol to determine the status of the channel, in step 240. If the data modem loses frame synch during these waiting periods, the process generally returns to step 220 in re-seeking frame synchronisation.
If the data modem has acquired (or maintained) frame synch when receiving a new PDU in step 210, the process moves immediately to waiting for the end of a microslot in step 240. At the end of a slot, if the channel is determined as being ‘busy’ the data modem introduces a random back-off delay in step 250, and waits again for an end of slot period when the channel is ‘idle’. When an end of slot is subsequently found, and the channel is determined as being ‘idle’, the new PDU is transmit to the DBS, in step 260. Once the transmission is complete, the data modem returns to an ‘idle’ receive state.
In the RD-LAP data communication system, it is known that existing data (modulator/demodulator) modems regulate the traffic loading on the data communication system in order to minimise collisions between different transmitting data units. The regulation is achieved by the communicating modems inserting a fixed time interval, a so called ‘SDU time interval’, between two successive messages, as shown in relation to FIG. 3.
FIG. 3 shows a timing structure 300 for a data modem's transmission. The data modem transmits a first Service Data Unit (SDU) 310 of say, 512 bytes. The first SDU 310 includes a warm-up field 315, followed by a preamble 320, a frame sync. 325 and station identifier (ID) 330. The data message 335 is then sent to a HOST, followed by a synchronisation period of 150 msec. A fixed SDU time interval 340 of at least one second is then inserted by the data modem before the second SDU 350 can be sent.
The inventors of the present invention have appreciated that the use of the fixed SDU time interval 340 effectively limits the throughput on the inbound data communication channel, even though it reduces, to some degree, the possibility of collisions. Thus, the SDU time interval 340 is the dominant factor in the timing of data modem transmissions.
However, the insertion of an SDU time interval 340 does not completely prevent uncontrolled collisions for all situations. For example, too many data modems may be using the available communication resource. When this happens, the effect of the SDU time interval is minimal as more data communication units attempt to access ever few communication resources. Eventually, if the channel reaches an overloaded state, no benefit can be gained by employing an SDU time interval.
Furthermore, by inserting gaps in the transmission to reduce the risk of collision, the data throughput per data communication unit (modem) is reduced. An example of a data message that may be affected by the reduced throughput could be, say, from a police officer in a car to a control centre. The message might be a query about the registration details of a particular car which has been stopped or is being followed by the police. A return message will contain the reply to the Police Officer's request for information. Clearly, in such situations, speed and reliability of the communication is paramount.
However, in practice as a result of employing an SDU time interval, the utilisation of the inbound channel does not exceed 20% for any single modem. This level of performance is unsatisfactory for such time-critical communication.
Thus, the SDU time interval mechanism has the disadvantage that it limits inbound throughput to an unacceptable level and is also not a failsafe mechanism for preventing uncontrolled collisions if the channel is overloaded. A need therefore exists for a mechanism to improve the channel loading of a data communication system, wherein the abovementioned disadvantages may be alleviated.